No. 6(l92l) COMMON MOLLUSCS OF SOUTH INDL\ 193 



The HOLY-WATER Clams {Tridacnidae) are so called because of 

 the use to which their huge ribbed valves are put in Roman Catholic 

 churches in Europe. They grow to a larger size than any other 

 bivalve, the shell attaining in exceptional cases a weight of over 

 500 lb. The substance of the shell is white and porcellanous and 

 very thick and strong. A single adductor of enormous power 

 controls the opening and closing of the valves. The smaller species 

 usually bore into dead coral with the help of a great mushroom- 

 shaped foot, forming roomy burrows ; others, often of great size, are 

 non-boring in habit and either lie loose among coral blocks or cling 

 to the rock by means of the foot. Stories are told of the great power 

 of their grip and pearl divers in the South Seas dread as one of 

 the greatest dangers of their calling, the possibility of setting a 

 foot within the gape of a Tridacna shell. The valves are deeply 

 corrugated with a number of very bold ridges. They live associa- 

 ted with corals and are most numerous and of greatest size in 

 the South Seas, the eastern section of the Malay Archipelago and 

 on the Great Barrier Reef of Australia. In our Indian seas, we 

 find them among the coral reefs of Pamban and the Gulf of Mannar ; 

 they are especially abundant at the Laccadives. To see the 

 gorgeous colouring of the mantle frilling as these molluscs lie 

 with their shells agape in a clear pool is one of the many wonders 

 of a coral reef; the glory of Joseph's coat was as naught compared 

 with the sparkling loveliness of the iridescent blues and pur- 

 ples and oranges of the mantle filaments protruding from between 

 the open valves. 



The next family, the Chainidac, has several essential morpho- 

 logical features common with the cockles and tridacnas ; but 

 unlike them its members live their adult life with one valve, the 

 left, cemented to stones, corals and shells. The Chamas are all 

 small and may be taken for the Thorny Cockles {Spoudylidae) if 

 the form of the hinge be not noticed. In Chama, the ligament is 

 external and not in a pit within, and there are fewer hinge teeth, 

 two in one valve and one only in the other. The brilliant florid 

 colouring of the spondyles is also absent, Chama being a pale 

 insignificant little shell. 



Of the Psamnwbiidac, the lovely Snlctdliua dip/105 is the best 

 known representative on our coasts. It is a long, oval, purple 

 coloured shell, rounded in front and narrowed behind to a some- 

 what pointed form ; in life the colour is masked by a 'smooth dark 

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